Stefanie Sergeant

Thermal oxidation of two-dimensional (2D) transition-metal dichalcogenide materials is attracting interest for gate-stack formation in advanced electronic devices. However, the oxidation kinetics and mechanism of thermal oxidation of 2D WS2 crystals grown by the industrially relevant metal-organic chemical vapor deposition technique are not yet understood. In this paper, we investigate the thermal oxidation of monolayer-thin triangular WS2 isolated crystals on a SiO2 substrate by O2. In a temperature window between 300 and 500 °C, we observe that the oxidation propagates laterally from the edges to the center of the WS2 crystals. The low oxidation rate enables control of the lateral oxidation length at the nanometer scale. As such, we create WS2/WO3 lateral heterostructures with a WO3 lateral oxidation length that linearly depends on the oxidation time, consistent with a well-controlled reaction-limited process with an oxidation rate of 0.03 nm/s at 375 °C. The Arrhenius energy barrier for oxidation may suggest that S-vacancies near the WS2/WO3 line boundary initiate the oxidation. Finally, we investigate the atomic layer deposition of a high-k dielectric on the WS2/WO3 lateral heterostructures and observe selective deposition of Al2O3 on the WO3 area, while no Al2O3 deposition is detected on the WS2 area of the heterostructure. The obtained insights into the thermal oxidation kinetics and selectivity are relevant to creating various types of heterostructures with 2D materials and gate dielectrics for highly scaled nano-electronic devices.

Metal-organic chemical vapor deposition of transition metal dichalcogenides is often accompanied by gas-phase reactions, which may lead to metal-based cluster formation in the gas phase and reduced deposition on the substrate surface. This study demonstrates that the cluster formation can be suppressed by keeping the residence time of metal precursor molecules in the reaction chamber significantly shorter than the typical collision time between such molecules. The rate of cluster formation was quantified by a single dimensionless parameter, the reduced residence time τr, which is the ratio of the average residence time to the intermolecular collision time. From an experimental study of WS2 metal-organic chemical vapor deposition using W(CO)6 and H2S precursors, the deposition rate is mass-transport limited at low W(CO)6 concentrations and process pressures, corresponding to τr ≪ 1. However, for τr ≥ 1, the deposition rate drops sharply, indicating cluster formation.

Pockels materials are notable for their strong electro-optic interaction and rapid response times and are therefore used extensively in optical communications. However, at cryogenic temperatures, Pockels coefficients are reduced in many materials optimized for room-temperature operation, which is a major hurdle for emerging quantum technologies. Here, we show that strontium titanate (SrTiO ₃ ) can be engineered to exhibit a Pockels coefficient of 345 picometers per volt at 20 hertz at cryogenic temperatures, a value twice as high as any other thin-film electro-optic material. By adjusting the stoichiometry, we were able to increase the Curie temperature and realize a ferroelectric phase yielding a high Pockels coefficient, so far with limited optical losses of decibels per centimeter. Our findings position SrTiO ₃ as a promising material for cryogenic quantum photonics applications.

Ferroelectrics show promise for low‐power, non‐volatile memory technologies. However, material challenges in state‐of‐the‐art ferroelectric hafnates and the high coercive fields required limit their application in devices. Scaling of other candidate materials is challenging, often requiring epitaxial single‐crystalline growth using specialised substrates. Here, ferroelectricity is demonstrated in polycrystalline BaTiO₃ films at 10 nm thickness on Si substrates. They exhibit the highest reported remnant polarization for polycrystalline layers, 13 µC cm⁻², a value that is competitive with the epitaxial BaTiO₃ state‐of‐the‐art. This is realised by introducing a novel conductive oxygen barrier, platinum silicide, which also offers strain enhancement of the ferroelectricity. Moreover, it is demonstrated that these layers can be positioned in device‐like stacks whilst maintaining ferroelectricity at 10 nm. The findings of polycrystalline perovskite ferroelectric growth in stack configurations akin to those in production flows paves the way for high performance perovskites with greater material complexity.

Two-dimensional transition metal dichalcogenides (2D TMDs), such as MoS2 and WS2, have emerged as promising channel materials for future generation transistors. However, carbon-based surface contaminants pose a significant challenge in the formation of high-quality metal–oxide–semiconductor gate stacks for 2D TMDs. Carbon-based surface contaminants are known to be present even on directly grown 2D TMDs that have not been in contact with polymers. These organic contaminants affect precursor adsorption during atomic layer deposition (ALD) of gate dielectrics on 2D TMDs and as such the 2D-dielectric interface. This study examines the effectiveness of predeposition annealing in mitigating carbon-based contaminants while maintaining the integrity of a directly grown WS2 monolayer on a SiO2 substrate. We show that a WS2 monolayer on a SiO2/Si substrate remains stable during vacuum annealing at temperatures up to 400 °C. Water contact angle measurements and x-ray photoelectron spectroscopy confirm that the surface concentration of carbon starts to decrease at 150 °C. Thermal anneal improves the surface coverage of Al2O3 for both conventional chemisorption-based ALD and physisorbed-precursor-assisted ALD processes by facilitating more effective Al2O3 nucleation on the WS2 monolayer. The impact of predeposition anneal on the Al2O3 growth behavior in both processes can be explained by changes in surface contaminant levels. Our results underscore the importance of surface pretreatment in dielectric deposition on 2D TMDs and demonstrate that predeposition anneal is an effective method to enhance ALD-based dielectric deposition on directly grown 2D TMDs.

In semiconductor processing and metrology, Raman spectroscopy is a valuable characterization tool because of its nondestructive nature, high throughput, and versatility in terms of parameter sensitivity. However, with the miniaturization of semiconductor devices, the inherent diffraction limit of the optical technique becomes a roadblock. In order to re-enable the strengths of Raman spectroscopy at the nanometer scale, we exploit polarization-induced enhancement effects that focus the excitation light into the region of interest, without the need for external probes or particles. This allows the detection of structures with dimensions far smaller than the excitation wavelength, unlocking the strengths of Raman spectroscopy at the nanoscale for, e.g., stress and composition measurements. Moreover, under these conditions the experiment probes the totality of the materials stack and we show how this transforms the technique into a volumetric and geometric measurement. The result is a completely new application domain for Raman spectroscopy as a critical dimensional metrology toolkit for a wide variety of semiconducting and metallic materials.

We introduce a new scanning probe microscopy (SPM) concept called reverse tip sample scanning probe microscopy (RTS SPM), where the tip and sample positions are reversed as compared to traditional SPM. The main benefit of RTS SPM over the standard SPM configuration is that it allows for simple and fast tip changes. This overcomes two major limitations of SPM which are slow data acquisition and a strong dependency of the data on the tip condition. A probe chip with thousands of sharp integrated tips is the basis of our concept. We have developed a nanofabrication protocol for Si based probe chips and their functionalization with metal and diamond coatings, evaluated our probe chips for various RTS SPM applications (multi-tip imaging, SPM tomography, and correlative SPM), and showed the high potential of the RTS SPM concept.

BaBiO3 has attracted a lot of research attention since it was discovered as the parent compound for the high-Tc superconducting BaPbxBi1–xO3 and Ba1–xKxBiO3. In its pure state, BaBiO3 is an insulator due to the presence of a breathing distortion of the BiO6 octahedra. The distortion is attributed to the valency of Bi in the compound being charge-ordered in the form of Bi3+ and Bi5+ along the lattice, resulting in alternating expanded or contracted BiO6 octahedra. The interaction between the electronic properties and the thickness of the thin film is crucial to study. We conducted a thorough study to investigate the effect of the thickness reduction of BaBiO3-δ grown on SrTiO3-buffered Si substrates on the optical properties as well as the Bi electronic structure of the thin films. We conclude that modifications in the valency of Bi in the ultra-thin film regime result in an optically conducting layer.